You can save emails as a document in a Task under the Opportunity. Any user with access to your GrantHub organization will be able to open these files as long as they have Outlook enabled on their computer.
In Outlook, click on the email you would like to save in GrantHub, then click on File and click on Save As. Choose the folder you would like to save this in. Also choose the 'type' you want to save the email as in the Save As Type option. Both 'Outlook Message Format - Unicode' and 'Text' file types can be saved as documents in GrantHub. If you save as 'Outlook Message Format - Unicode', keep in mind that you will need to open those files in Outlook to read them.
To save in your Task, fill out all required information, click Save and then drag and drop your .msg file and save the Task again.